This invention relates to the circuit construction of a heterodyne receiver (including a super heterodyne receiver, double super heterodyne receiver, homodyne heterodyne receiver), and more particularly to correction of the amplitude of a local oscillation signal input to a frequency mixing circuit (including a single-balanced type mixer, double-balanced type mixer, single transistor type mixer, single FET type mixer) for processing a high-frequency signal.
FIG. 1 shows one example of the circuit construction of a conventional heterodyne receiver.
A local oscillator 15 constructed by a phase-locked loop (PLL) circuit includes a voltage-controlled oscillator (VCO) 7, OSC buffer amplifier 8, prescaler buffer amplifier 9, and phase comparator circuit 10. In the local oscillator 15, the VCO 7 creates a local oscillation signal in synchronism with a signal input from a frequency selection circuit 11. The buffer amplifier 8 amplifies the local oscillation signal supplied from the VCO 7 and supplies the amplified signal to a frequency mixing circuit (which is hereinafter simply referred to as a mixer) 4.
A received high frequency input signal Si is supplied to a band-pass filter (BPF) 2a via an automatic gain control (AGC) circuit 1 whose gain is controlled based on an output of a detector 6. An output signal of the band-pass filter 2a is amplified by a high frequency amplifier (RF Amp) 3 and input to the mixer 4. In the mixer 4, the amplified high frequency input signal and the local oscillation signal from the buffer amplifier 8 are mixed and an intermediate frequency signal is output. The intermediate frequency signal is amplified by an intermediate frequency amplifier (IF Amp) 5 and input to the detector 6 via a band-pass filter 2b which passes the intermediate frequency signal.
FIG. 2 shows an example of a circuit of the mixer 4 and the buffer amplifier 8. In the circuit of FIG. 2, a double-balanced type mixer circuit is used as the mixer 4. The amplitude level of a signal output from the buffer amplifier 8 to the double-balanced type mixer 4 is determined by the circuit construction of the buffer amplifier 8 and the circuit constants thereof.
In the buffer amplifier 8, the load impedance as viewed from the output terminal of an emitter follower circuit, that is, from a point "b" in FIG. 2 is a combination of the input impedance of a mixer circuit 13 and the input impedance of the prescaler buffer amplifier 9 and is generally low. Therefore, the dynamic ranges of transistors Q3, Q4 of the emitter follower circuit are narrowed and the output waveform of the buffer amplifier 8 is clipped. FIG. 3 shows a waveform (indicated by a solid line) at the circuit point "a" and a waveform (indicated by broken lines) at the circuit point "b" which is the output terminal of the emitter follower circuit. The waveform at the circuit point "b" corresponds to a waveform obtained by clipping the waveform at the circuit point "a".
Since the heterodyne receiver acts as a distributed constant circuit in the high-frequency bandwidth, it becomes necessary to take the parasitic capacitor and inductance into consideration as the impedance.
FIG. 4 shows the relation between the amplitude level (voltage) of the local oscillation signal input to the mixer 4 and the conversion gain of the mixer 4 and IP3 characteristic (third inter-modulation distortion). As is clearly seen from FIG. 4, as the amplitude level of the local oscillation signal becomes higher, the conversion gain becomes larger to some extent, but if the amplitude level of the local oscillation signal is further enhanced, the conversion gain becomes smaller. Further, as the amplitude level of the local oscillation signal becomes lower, the modulation distortion becomes smaller. The optimum amplitude level of the local oscillation signal lies between the circuit points "a" and "b" in FIG. 4, for example.
FIG. 5 shows the relation between the frequency of a high-frequency input signal and the conversion gain of the mixer when V1, V2, V3, V4 (V4&lt;V3&lt;V2&lt;V1) are used as parameters. As shown in FIG. 5, the rate of a variation in the conversion gain with a variation in the high-frequency input signal is changed by changing the amplitude level of the local oscillation signal as a parameter. In order to attain a preferable conversion gain, it becomes necessary to set the amplitude level of the local oscillation signal higher as the frequency of the high-frequency input signal is set higher.
At the time of circuit design, it is necessary to determine the driving ability of the emitter follower circuit of the buffer amplifier 8 so as to set the amplitude level of the local oscillation signal into a region between "a" and "b" of FIG. 4 by taking the above-described fact into consideration and carrying out complicated calculations. The driving ability is determined based on the circuit construction and the circuit constants. Since the driving ability cannot be changed once it is determined, the amplitude level of the local oscillation signal from the buffer amplifier 8 is also fixed.
Further, even if the amplitude level of the local oscillation signal is set to an optimum value at a certain frequency, the reactance component changes and the load impedance changes if the frequency is changed, and therefore, the optimum value of the amplitude level is changed. For this reason, when the frequency of the local oscillation signal is wide band, it becomes necessary to attain the amplitude level of "a" in FIG. 4 in the high frequency range and attain the amplitude level of "b" in FIG. 4 in the low frequency range, and in order to meet the above requirement, it is necessary to make a careful study at the time of circuit design.
Because of the circuit construction, it is necessary to fix the amplitude level of the local oscillation signal input to the mixer 4 and it is impossible to change the same, and therefore, the current cannot be reduced and the power consumption cannot be lowered.
Further, even if the intensity of the high-frequency input signal which the heterodyne receiver receives becomes higher, the amplitude level of the high-frequency input signal is controlled by the AGC circuit so as to be set to a constant value, and therefore, it becomes impossible to reduce the current consumed in the mixer 4.